Vestibular, proprioceptive and visual inputs are integrated by the central nervous system to produce appropriate ocular motor and postural responses. Cervical afferents provide unique information that enable head-referenced signals from the visual and vestibular systems to be transformed into trunk-referenced signals. Many patients with balance disorders note an association between a worsening of their dizziness and neck discomfort. The basis for this association is unknown. The long-term objective of this research is to develop a rational approach to the diagnosis of cervicogenic dizziness and to the assessment of cervical influences on vestibular disorders. Each of the three specific aims of this exploratory research proposal relates to improving our understanding of cervical influences on balance. In particular, the goal of the experiments proposed herein is to generate testable hypotheses regarding objective measures of cervical influences on vestibulo-ocular and vestibular-spinal responses, and on visually induced postural sway. The specific aims of this proposal are to explore three different aspects of the cervical influences on balance: 1. The influence of roll head-on-trunk position on the orientation of Listing's plane with respect to the head, 2. The influence of yaw head-on-trunk position on postural sway induced by sinusoidal interaural galvanic vestibular stimulation, and 3. The influence of yaw head-on-trunk position on postural sway induced by naso-occipital optic flow. To address specific aim #1, three-dimensional eye positions will be monitored during various combinations of head and trunk roll tilt using dual-scleral search coils. To address specific aim #2, postural sway induced by sinusoidal, bipolar, binaural galvanic vestibular stimulation during several yaw head-on-trunk positions will be measured. To address specific aim #3, postural sway will be measured in response to naso-occipital optic flow within a virtual reality environment during several yaw head-on-trunk positions.